1. Field of the Invention
This invention relates generally to an electronic system and method for controlling ignition timing in an internal combustion engine and more particularly concerns a closed loop electronic system and method for controlling the spark timing and knock condition in each cylinder separately of a multicylinder internal combustion engine.
2. Description of the Prior Art
Under normal conditions, ignition timing in a particular cylinder is controlled by the distributor in a spark ignition internal combustion engine so that the spark occurs several degrees before the piston in the cylinder reaches its top dead center position or point of minimum combustion volume. This allows the ignition lag and the propagation of a flame front in the combustion chamber to complete the combustion of the compressed gas mixture of air and fuel so that the maximum pressure of combustion will occur so as to maximize economy and/or power within exhaust emission constraints.
The usual automatic spark control device connected with the timing mechanism is often so arranged that the spark is advanced to a considerable extent as the speed of the engine increases. In fact, in some makes of automobile engines, this advance in the spark in one cylinder could be as much as 50 or 60 degrees before the position of the piston in the cylinder reaches its top dead center position. This is necessary because, even though the speed of the engine increases, the rate of combustion and flame propagation within the engine is nearly constant and therefore does not correspondingly increase. Hence, in order to insure complete combustion at the right point, the spark must be automatically advanced. This advance of the spark often, in some types of engines and particularly when a low octane gasoline is used or there has been considerable carbon deposit, may produce knock or detonation.
Knock or detonation is the pressure oscillations of the gases in the combustion chamber in spark ignition engines caused by the self-ignition of the remaining air-fuel mixture in this chamber before this mixture can be burned completely by propagation of a flame front. If the degree of compression of the fuel charge at or immediately before ignition exceeds the ability of the fuel to burn completely by propagation of a flame front, an audible detonation or knock may occur. The audible knock is due to the transmission of vibrations through the rest of the engine and perhaps a resonance response in certain engine parts.
It is well known that knock or detonation in engine operation can usually be decreased by a knock control system which delays the ignition timing to a value at which the fuel will burn without detonation. This prevents complete combustion from taking place until after the piston has passed its top dead center position or the point of minimum combustion volume.
It must be recognized that it may be desirable to maintain the spark timing at the point of maximum advance consistent with operation of the engine with knock at an acceptably low level. Delays in spark ignition introduced by a knock control system detract from the power developed by the engine. Thus, engine performance may suffer when knock is eliminated by spark delay and it may be desirable to have a small amount of knock present in order to insure that engine performance and fuel economy are maintained at a high level. On the other hand, excessive knock is aesthetically objectionable and can damage an engine. Thus, it is desirable to control engine operation so that a certain amount of knock is allowed or even maintained but so that the knock is not permitted to become loud enough to be objectionable to the vehicle occupant or to cause engine damage.
Only a knock control system which entails automatic and continuous control of the knock condition during normal operation of an internal combustion engine permits. maximization of the benefits of knock control. Keller et al., U.S. Pat. No. 3,822,583, which issued July 9, 1974, disclose one such system and method which employs closed loop electronics for automatically controlling the timing of the spark ignition and thereby the knock condition in an internal combustion engine during operation. The device disclosed includes means for detecting engine vibrations during an engine cycle resulting from engine knock and normal engine operation and for producing a composite signal which has an engine knock component and a normal engine performance component and which is proportional to the intensity of a portion of said engine vibrations having frequencies which include at least one frequency of said knock vibrations, the composite signal having an engine knock component and a normal engine operation component; means for generating a background signal which corresponds to the normal engine operation component of the composite signal; means for measuring the difference between the composite signal and the background signal to obtain the knock component of the composite signal and for generating a knock signal in direct relation to the knock component of the composite signal; and means for adjusting, if the knock signal is different from a predetermined level therefor, the spark ignition timing in all of the cylinders simultaneously and uniformly during at least one successive engine cycle, the adjustment being such as to reduce the difference between the knock signal and the predetermined level therefor. The entire disclosure of U.S. Pat. No. 3,822,583 is specifically incorporated herein by reference.
Subsequently, a closely related device and method were disclosed by J. H. Currie, D. S. Grossman and J. J. Gumbleton in two papers, one entitled "Knock Control for Energy Conservation" and presented at an ACS meeting on Conservation at Miami Beach in Sept., 1978, and a second entitled "Energy Conservation with Increased Compression Ratio and Electronic Knock Control" and presented in the SAE Technical Paper Series in Detroit on Feb. 26-Mar. 2, 1979, and by T. F. Wallace in a paper entitled "Buick's Turbocharged V-6 Powertrain for 1978" and presented in the SAE Technical Paper Series in Detroit on Feb. 27-Mar. 3, 1978.
However, it must be recognized that although the knock level in one cylinder may be objectionably loud, the knock level in the other cylinders may not be objectionably loud and, in fact, the knock level in one or more of these other cylinders may even be lower than the level desired to maximize engine performance and fuel economy. Under such conditions, it would result in a loss of vehicle performance and fuel economy if spark ignition were delayed in all of the cylinders merely to delay the ignition spark in the one cylinder having an objectionably loud knock. Thus, when an engine is knocking at an objectionable level, it is desirable to maximize vehicle performance and fuel economy by delaying the spark ignitions in only those cylinders where detonation is occurring at an objectionable level. Furthermore, even if several or all of the cylinders are knocking at an objectionable level, such cylinders may not be knocking to the same extent, and thus the spark ignitions in such cylinders should be delayed by different degrees in order to maximize engine performance and fuel economy. Moreover, it may be desirable to advance the spark ignition in each cylinder where the knock level is lower than that desired to maximize engine performance and fuel economy. The aforedescribed prior art closed loop electronic systems and methods do not permit such selective control of the timing of the spark ignitions in each cylinder separately.